In eukaryotic cell culture systems, the culture of the cells is generally under conditions of controlled pH, temperature, humidity, osmolarity, ion concentrations, and exchange of gases. Regarding the latter, oxygen and carbon dioxide (CO2) are of particular importance to the culturing of cells. In a typical eukaryotic cell culture system, an incubator is provided in which CO2 is infused to maintain an atmosphere of about 5% CO2 within the incubator. The CO2 interacts with the tissue culture medium, particularly its buffering system, in maintaining the pH near physiologic levels. Conventional cell culture containers comprise tissue culture flasks, tissue culture bottles, and tissue culture plates. Entry of CO2 from the incubator atmosphere into a tissue culture plate generally involves a loosely fitting cover which overhangs the plate in excluding particulate contaminants from entering the plate chamber(s), but allows gas exchange between the incubator atmosphere and the atmosphere within the tissue culture plates. Similarly, for a tissue culture flasks or bottle, a loosely fitting cap excludes particulate contaminants from entering the chamber of the flask or bottle, but allows gas exchange between the incubator atmosphere and the atmosphere within the flask or bottle. More recently, a cap is provided with a gas permeable membrane or filter, thereby allowing for gas exchange with a tightly fitting cap.
In addition to CO2, the culturing of cells is dependent upon the ability to supply to the cells a sufficient amount of oxygen necessary for cell respiration and metabolic function. The supply of oxygen for cell respiration in conventional cell culture containers is in the header space of the container, e.g., the void space in the container that is above the surface of the tissue culture medium. Efforts to increase oxygen concentration to the cultured cells includes mechanical stirring, medium perfusion or aeration, increasing the partial pressure of oxygen, and/or increasing the atmospheric pressure. Thus, in conventional cell culture containers the volume or surface provided for gas exchange, as relative to the volume or surfaces of the whole container, is either inefficiently used and/or results in limiting the rate of gas exchange or in the equilibration of gases. This is even more noticeable in small-scale cultures (15 ml or less) in which rate of cell growth, cell densities, and total cell numbers, are frequently low due to space, surface area, and gas exchange limitations. There is also evidence that suboptimal oxygen levels across precursor tissues in vitro result in a lower degree of differentiation.
Varying levels of oxygen in cultured embryonic stem cells, for instance, determine whether they will proliferate or differentiate. A clear relation between oxygenation and differentiation has also been observed in endothelial and mesenchymal stem cells. In another in vitro system, we have shown that pancreatic beta cell differentiation in vitro is greatly enhanced by oxygen. This is consistent with the observation that the second and most significant wave of beta cell specification during embryonic development (secondary transition) is concurrent with the initiation of blood flow within the pancreatic buds. There is, therefore, a need in the art to provide tissue culture systems wherein oxygen delivery is enhanced, or adjusted depending on the culture setting, proliferation, differentiation and/or viability.